An angular velocity sensor disclosed in patent document 1 is described hereinafter with reference to FIG. 6 and FIG. 7 as one of conventional electronic components. FIG. 6 shows a perspective view of a detecting element of the conventional angular velocity sensor, and FIG. 7 shows a sectional view cut along line 7-7 in FIG. 6.
As shown in FIG. 6, the conventional angular velocity sensor comprises detecting element 2 shaped like a tuning fork and a signal processing circuit (not shown) which processes a signal supplied from detecting element 2 for calculating an angular velocity.
Detecting element 2 is formed of a pair of arms 4 confronting each other and supported by base 6, thereby shaping like a tuning fork. Base 6 in parts is mounted on a substrate. The pair of arms 4 comprises the following elements:                driving electrode 8 for supplying a driving signal which drives arms 4 to piezoelectric unit 20;        sensing electrode 10 for outputting an angular velocity signal generated by an angular velocity applied to detecting element 2; and detecting electrode 12 for detecting a driven status of        detecting element 2, thereby outputting a detection signal.        
Two driving electrodes 8 sandwich one sensing electrode 10 therebetween with respect to the face-to-face direction of arms 4 confronting each other. Detecting electrode 12 is placed closely to the border between arms 4 and base 6.
As shown in FIG. 7, two arms 4 is formed of a silicon substrate including two layers, i.e. silicon layer 14 and silicon oxide layer 16 that is formed by oxidizing the surface of silicon layer 14. On this silicon substrate, driving electrodes 8 and sensing electrodes 10 are respectively layered via first adhesive layers 18. Each one of driving electrodes 8 and sensing electrodes 10 is formed of lower electrode 22, upper electrode 24, and piezoelectric unit 20 existing between lower and upper electrodes 22 and 24. Second adhesive layer 26 is formed between piezoelectric unit 20 and upper electrode 24 for bonding these two elements together.
First adhesive layer 18 is formed of Ti-layer, and lower electrode 22 is formed of Pt—Ti layer including Ti and Pt as a main ingredient. Piezoelectric unit 20 is formed of two layers, i.e. orientation control layer 28 made of lead-titante based material, and PZT layer 30 made of lead zirconate titante. Second adhesive layer 26 is formed of Ti-layer, and upper electrode is formed of Au-layer.
Detecting element 2 discussed above can be manufactured with the following method: First, layer first adhesive layer 18 on a wafer-like silicon substrate, then layer lower electrode 22 onto first adhesive layer 18, and then layer orientation control layer 28 onto lower electrode 22, then layer PZT layer 30 onto orientation control layer 28, then layer second adhesive layer 26 onto PZT layer 30, and finally layer upper electrode 24 on second adhesive layer 26.
Second, provide the foregoing layers with dry-etching and wet-etching by using the photolithography method in order to form driving electrode 8, sensing electrode 10 and detecting electrode 12 in a given shape, and these electrodes are extended and routed, thereby forming signal paths 13.
Third, provide the foregoing product with a polarization process and an annealing process to stabilize PZT characteristics (polarization state) and secure other characteristics.
Next, provide the wafer-like silicon substrate with dry-etching to form multiple detecting elements 2 shaped like tuning forks, and then dice this substrate into discrete detecting elements 2.
The PZT characteristics of the foregoing detecting element 2 are shown in FIG. 8 with dotted line 101, and it tells that the reaction between Pb in PZT layer 30 and Ti in second adhesive layer 26 lowers the piezoelectric constant. A temperature-rise during the annealing process invites this reaction.
FIG. 9 shows variation with time in voltage at a base point of an angular velocity sensor during a high temperature. Dotted line 102 tells the following phenomenon: A longer time is taken by the angular velocity sensor employing the detecting element 2 discussed above during the high temperature (working on 5V at 125° C.), a greater change occurs in the base point voltage where no angular velocity is applied.
FIG. 10 shows a change in an output voltage from the foregoing angular velocity sensor in response to a change in angular velocity. As dotted line 103 tells, a voltage at the base point, where no angular velocity occurs, shifts to the negative side, thereby incurring an error (A) deg/s, which degrades the accuracy of the angular velocity sensor.
The foregoing conventional structure have invited the problems as discussed above, i.e. the temperature-rise during the annealing process invites lowering the piezoelectric constant, or a greater change in the base point voltage, where no angular velocity is applied, is caused by a longer working time when the sensor works at a high temperature. These problems degrade the characteristics of the sensor.    Patent Document 1: Unexamined Japanese Patent Application Publication No. 2002-257549